Steam generating or utilizing systems, such as steam boilers and turbines, frequently operate with superheated steam, or vapor at a higher temperature than the saturation temperature at a particular pressure. Because superheated steam can reach temperatures which damage the turbine or the superheater itself, it is necessary to maintain close control over the superheat temperature. Although there are several known techniques for controlling superheat temperature, one favored technique is to reduce the superheat temperature by injecting a water spray into the steam line. This water spray reduces the superheat temperature by the amount of heat required to raise the injected mass of water to the vaporization temperature and then vaporize the water.
Because of the need for close control of superheat temperatures, water spray desuperheating must be capable of injecting selectably variable amounts of water into the superheated steam. Thus, it is known that water-spray desuperheaters should be capable of throttling modulating flow over a range from the maximum water-injecting opening down to a desired minimum flow opening.
The goal of water-injecting desuperheaters is to maximize the temperature reduction obtained for a given amount of water added to the superheated steam. To attain that goal, the desuperheater should introduce water in a geometric pattern providing a homogenous flow while maximizing the surface area of the water, thereby decreasing the time required to vaporize the desuperheating water. Moreover, the desired geometric pattern of water introduced to the steam flow should be maintained at all possible throttled settings of the desuperheating valve, from maximum flow down to minimum flow, so as to provide a relatively constant correlation between the amount of water introduced at a particular setting of the desuperheater valve, and the desuperheating effect obtained at that setting.
Water-spray desuperheaters of the prior art generally have not maintained the desired geometric flow pattern over all positions of the valve, particularly at the lower-volume throttled positions. Such valves generally provide an orifice sized to introduce the maximum desired volume of desuperheat water, and this orifice can be configured to provide the desired geometric pattern of water for maximizing surface area of the water at or near the designed maximum flow rate. Such valves are throttled by partially plugging or blocking the orifice to reduce the maximum flow. These reduced flow rates are generally accompanied by a reduction in the velocity of water introduced to the superheated steam from the throttled orifice, to a point where the flow velocity becomes too low to maintain the desired geometric pattern of water dispersion provided at less-throttled positions of the valve. Consequently, water injecting desuperheaters of the prior art tend to operate less efficiently at reduced amounts of water flow.